Plants harbor a diverse assemblage of microbial symbionts, including bacteria and fungi in leaves and roots.  The ecological consequences of many of these microbes remain unknown, particularly in natural ecosystems.  In grasses, class I endophytic fungi grow systemically in above-ground plant tissues and can have important effects on tri-trophic interactions.   Grass endophytes have been well studied in a few host species due to their economic and environmental impacts in forage and turf production.   However, the ecological function of endophyte symbiosis remains unresolved for the vast majority of grass species.  Here, we examined the influence of endophyte presence/absence as well as endophyte quality (genotype) on insect herbivore and arthropod predator/parasitoid communities in the well-studied symbiosis between tall fescue grass (Lolium arundinaceum) and its specialist endophyte, Neotyphodium coenophialum.  In addition, to provide a native grass-endophyte comparison, we examined whether endophytes mediated tri-trophic interactions between aphids and parasitoids in Poa autumnalis.  In both cases, endophyte presence (or genotype) was experimentally manipulated to disentangle the influence of endophyte symbiosis from the effect of plant genotype.  In the spirit of this year’s focus on global warming, we also applied manipulations of water availability, and we interpret results in the context of predicted climate changes.  

Results/Conclusions

Experiments revealed diverse ecological consequences of endophyte symbiosis on arthropod community structure.  Both endophyte presence and endophyte genotype altered arthropod composition, herbivore abundance, and the diversity of predators and parasitoids in the community.  Some insect clades were particularly sensitive to endophyte symbiosis; for example, Orthoptera showed a strong response to endophyte genotype, and Coleoptera were suppressed by both endophyte genotypes tested relative to endophyte-free plots.  Results from the native symbiotum between Poa autumnalis and Neotyphodium revealed the complexity of tri-trophic level consequences.  Presence of the endophyte reduced aphid abundances and increased parasitism rates, revealing a synergism between the bottom-up effect of symbiosis and the top-down effect of parasitoids in decreasing aphid abundances.  However, altered levels of water availability, a likely component of climate change, influenced the strength of the tri-trophic effects of endophyte symbiosis.  Our results demonstrate that the community-level consequences of endophyte symbiosis are not limited to managed, agronomic ecosystems and can depend on environmental factors, such as water gradients.  Symbionts contribute an additional, but often overlooked, layer of diversity in ecosystems, and their inclusion in ecological research may help increase the realism of both experiments and theory.